Refrigerant system with cascaded circuits and performance enhancement features

ABSTRACT

An improved refrigerant system incorporates at least two circuits arranged in a cascaded relationship. Preferably, the upper circuit utilizes a hydrocarbon refrigerant and preferably the lower circuit utilizes CO 2  refrigerant. Preferably, the CO 2  circuit mainly operates in a subcritical region. To improve the efficiency and capacity control of the cascaded refrigerant system, at least one of the circuits is equipped with performance enhancement features such as, for example, an economized function provided by a flash tank or economizer heat exchanger. Additional enhancement features can also include a liquid-suction heat exchanger and bypass function.

BACKGROUND OF THE INVENTION

This application relates to refrigerant systems with at least twocascaded circuits, and more particularly, to cascade refrigerant systemswith performance enhancement features.

For instance, two distinct refrigerants can be utilized in each of thetwo circuits, with a hydrocarbon refrigerant utilized only in the upperstage circuit and another refrigerant utilized in the lower stagecircuit. The hydrocarbon type of refrigerant can be, for example,propane or isobutene refrigerant. Since the upper stage circuit can belocated outside of the enclosed conditioned compartment, it would offeran advantage of locating the flammable refrigerant also outside of theenclosed space, which would mitigate flammability concerns of theserefrigerants. By utilizing the two cascaded circuits, the amount ofcharge in the upper stage circuit would be substantially reduced ascompared to a single circuit refrigerant system. Since the amount ofcharge in the upper circuit is minimized, the concerns for theflammability of the refrigerant in this circuit are also reduced.

Historically, conventional HFC and HCFC refrigerants such as R22, R123,R407C, R134a, R410A and R404A, have been utilized in air conditioningand refrigeration applications. However, recent, concerns about globalwarming and, in some cases, ozone depletion promoted usage of naturalrefrigerants such as R744 (CO₂), R718 (water) and R717 (ammonia). Inparticular, CO₂ is a promising natural refrigerant that has zero ozonedepletion potential and extremely low global warming potential of one.Thus, CO₂ is becoming more widely used as a replacement refrigerant forconventional HFC refrigerants. However, there are challenges for arefrigerant system designer with regard to utilizing CO₂. Due to its lowcritical point, CO₂ often operates in a transcritical cycle (rejectsheat above the two-phase dome or above the critical point) that hascertain inefficiencies associated with this heat rejection process.Therefore, refrigerant systems utilizing CO₂ as a refrigerant do notalways operate at the efficiency levels of traditional refrigerantsystems.

One of the approaches to overcome the deficiencies of the CO₂refrigerant is to utilize a cascaded design of the refrigerant system.For example, each of the two cascaded circuits can be charged with theCO₂ refrigerant. In this case, the system can be designed in such a waythat each circuit would have a lower pressure differential across thecircuit, than if only a single circuit refrigerant system was utilized.By reducing the pressure differential for each cascaded circuit thereliability and efficiency of the compressors can be increased. Inanother approach, the lower stage circuit is charged with the CO₂refrigerant. Since only the lower stage circuit is charged with CO₂,this circuit would operate at much lower pressure as compared to asingle circuit refrigerant system (not cascaded) charged with CO2refrigerant. Propane or a like refrigerant would be utilized in theupper circuit. However, even after splitting a single circuit into twoindependent cascaded circuits, the refrigerant system designer is stillfaced with many challenges dealing with further improvements of thesystem efficiency and capacity control.

Various enhancement features are known to increase the functionality andperformance of refrigerant systems. As one example, an economizer cyclemay be incorporated into a refrigerant system for its performance boost.An economizer cycle operates to subcool a main refrigerant flow, anddoes so, in one variation, by tapping a portion of refrigerant from themain refrigerant flow and expanding this tapped refrigerant to someintermediate pressure. This expanded refrigerant is at a coolertemperature, and passes in a heat exchange relationship with the mainrefrigerant flow in an economizer heat exchanger. In a variation of theeconomizer cycle, a flash tank replaces the heat exchanger, where vaporand liquid refrigerant phases are separated, with the liquid flowcontinuing through the main circuit and the vapor flow injected into thecompression process at some intermediate pressure. In either variation,a vapor refrigerant is returned to the compressor.

Another enhancement feature is a refrigerant bypass function. In abypass function, at least a portion of partially compressed refrigerantis returned to a refrigerant suction line, allowing for unloading of therefrigerant system.

Still another enhancement feature is a liquid-suction heat exchanger. Ina liquid-suction heat exchanger, refrigerant downstream of an evaporatorpasses in heat exchange relationship with a refrigerant downstream ofthe condenser, allowing for additional subcooling and capacity increaseof the refrigerant system. In the past, these enhancement features wereassociated with a standard circuit, where the circuit had an evaporatorand gas cooler (or condenser).

However, none of the above-referenced enhancement features has beenincorporated into a cascaded refrigerant system. In cascaded refrigerantsystems, each of the cascaded circuits does not operate with anevaporator and gas cooler. Instead, the lower stage circuit has anevaporator and shares the common refrigerant-to-refrigerant heatexchanger with the upper stage circuit. The upper circuit has the gascooler and shares the same common refrigerant-to-refrigerant heatexchanger with the lower circuit. In other words, there is no evaporatorassociated with the upper circuit and there is no gas cooler associatedwith the lower circuit.

This invention provides additional design features enhancing thecascaded system performance and functionality to become comparable tothe traditional refrigerant systems for a wide spectrum of operating andenvironmental conditions as described in the main body of thisapplication.

SUMMARY OF THE INVENTION

In this invention, cascaded refrigerant circuits are incorporated into arefrigerant system design. As one particular application, an upper stagecircuit includes a hydrocarbon refrigerant, such as for example propaneor isobutene, which can be located outdoors. The upper stage circuit ispositioned in a cascaded relationship with a lower stage circuit, whichwould normally utilize the CO₂ refrigerant. The upper stage circuit ismainly located in the outdoor environment, while the lower stage circuitis normally located in the indoor environment. However, other locationswould also fall within the scope of this invention. As one of thefeatures of this invention, the lower stage inside CO₂ circuit operatesin a subcritical region while the upper stage outside cascaded circuitwould operate in a transcritical region if it was charged with the sameCO₂ refrigerant. The combination of the two circuits providesperformance enhancements for the supercritical region operation of theCO₂ circuit. To enhance the operation of the cascaded refrigerant systemat least one of the circuits can be equipped with the economized cycle,utilizing either economizer heat exchanger or flash tank arrangements.Additionally, or as a stand alone feature, at least one of the circuitscan be equipped with a liquid suction heat exchanger. Further, anunloading feature can be provided for one or both cascaded refrigerantcircuits.

These and other features of the present invention can be best understoodfrom the following specification and drawings, the following of which isa brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of prior art system

FIG. 2 generally illustrates a feature of this prior art.

FIG. 3 shows a first embodiment of the present invention.

FIG. 4 shows a second embodiment of the present invention.

FIG. 5 shows a third embodiment of the present invention.

FIG. 6 shows a fourth embodiment of the present invention.

FIG. 7 shows a fifth embodiment of the present invention.

FIG. 8 shows a sixth embodiment of the present invention.

FIG. 9 shows a seventh embodiment of the present invention.

FIG. 10 shows an eighth embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a prior art refrigerant system 20 incorporating twocascaded circuits 21 and 23. A lower stage circuit 23 includes acompressor 22 delivering a compressed refrigerant into arefrigerant-to-refrigerant heat exchanger 24. Heat exchanger 24 ispreferably positioned outside of an environment 32 to be conditioned.Refrigerant passes from the heat exchanger 24 through an expansiondevice 26, and to an indoor heat exchanger 28. As known, a fan 30 blowsair over external surfaces of the indoor heat exchanger 28 and deliversthat conditioned air into the environment 32. The lower stage circuit 23would normally be charged with a refrigerant that would operate in asubcritical region. One such refrigerant that can be used for thiscircuit would be CO₂ refrigerant that, while in the lower cascadedcircuit, would still be in the subcritical region. If this same CO₂refrigerant would have been used in the upper cascaded circuit, it islikely to operate at transcritical regime.

In the upper stage circuit 21, a compressor 34 compresses a refrigerantand delivers it to a second outdoor heat exchanger 36. A fan 38 blowsair over the heat exchanger 36. Refrigerant passes from the heatexchanger 36 downstream to an expansion device 40, and then back throughthe refrigerant-to-refrigerant heat exchanger 24 to the compressor 22.

FIG. 2 shows a P-h chart for the refrigerant system 20.

The upper stage circuit 21 can be charged with a hydrocarbonrefrigerant, and in particular, this refrigerant is disclosed as one ofpropane or isobutene. It is known that propane and isobutene have greatthermo-physical properties as refrigerants, however, they are bothpotentially explosive, and there are safety concerns to use them,especially in confined environments. By limiting hydrocarbon refrigerantapplications to the outdoor heat exchangers, the problem ofexplosiveness is significantly reduced. Further, by charging only theupper stage cascaded circuit 21 with the hydrocarbon refrigerant, therefrigerant system designer reduces the total amount of the hydrocarbonrefrigerant used within the refrigerant system 20, consequentlydecreasing the flammability risk from using hydrocarbon refrigerants.Moreover, by positioning the fans 38 in an optimum orientation withrespect to heat exchanger 36, any leakage or accidental discharge of thehydrocarbon refrigerant into the conditioned space can be directedtoward the outdoor environment, thus further minimizing risks ofexplosion.

The lower stage cascaded circuit 23 preferably operates in a subcriticalregion. Further, while it is disclosed that the upper stage cascadedcircuit 21 operates with a hydrocarbon refrigerant, the circuit 21 canoperate with other suitable refrigerants.

In disclosed embodiments, additional enhancement features are providedto allow the cascaded circuits to perform more efficiently.

As shown in FIG. 3, the upper stage cascaded circuit 100 is equippedwith an economizer function 102 that would increase the capacity andamount of subcooling to the main refrigerant flow for this upper stagecascaded economized circuit 100. Consequently, the performance of thelower stage cascaded circuit 101 is also enhanced, since the performanceof the refrigerant-to-refrigerant heat exchanger 104, that provides heattransfer interaction means between the upper stage cascaded circuit 100and the lower stage cascaded circuit 101 and serves as a condenser forthe lower stage cascaded circuit 101, is increased. An economizer heatexchanger 109 and an economizer expansion device 99 are shown.Therefore, the capacity and efficiency of the overall cascadedrefrigerant system shown in FIG. 3 is augmented. As an additionalenhancement feature, a bypass valve 106 can be installed to connect anintermediate pressure side 107 of the upper stage cascaded circuit 100to the suction pressure side 108 of this circuit. Selective opening ofthe bypass valve 106 provides the compressor unloading and capacitycontrol means for the upper stage cascaded circuit 100, and thereforefor the entire refrigerant system.

The economizer function 102 provided for the upper stage cascadedcircuit 100 by the economizer heat exchanger 109 in the FIG. 3embodiment can be also provided by a flash tank 112, as shown in FIG. 4,and an expansion device 199.

In general, the use of economized circuits improves the systemefficiency, but also provides capacity control by selectively engagingthese circuits. As known, there are many variations of the economizercycle schematics, all of which can benefit and are within the scope ofthe invention. Also, selectively opening and closing the bypass valvewould provide additional flexibility for the capacity control for thecascaded refrigerant system.

The upper stage cascaded circuit 100 can also be equipped with aliquid-suction heat exchanger (LSHE) 114, as shown in FIG. 5, once againfor the purpose of improving the capacity and amount of subcoolingachieved in this upper stage cascaded circuit 100, by transferring heatfrom the hot refrigerant in a refrigerant line 116 to the suctionrefrigerant vapor in a refrigerant line 108.

Furthermore, FIG. 6 shows another embodiment where the economizer heatexchanger 109 and the liquid-suction heat exchanger 114 features arecombined to achieve even further capacity and efficiency improvementsfor the upper stage cascaded circuit 100, and thus for the entirecascaded refrigerant system.

FIG. 7 represents another cascaded schematic, where an economizer heatexchanger 120 is incorporated into the lower stage cascaded circuit 101.As an illustration of the additional optional feature, this lower stagecascaded circuit 101 can also be equipped with an unloader valve 122,which would allow for bypass of a portion of refrigerant from anintermediate pressure side to suction pressure side.

FIG. 8 shows yet another cascaded schematic where a flash tank 130 isincorporated into the lower stage cascaded circuit 101.

FIG. 9 shows still another cascaded schematic where a liquid-suctionheat exchanger 132 is incorporated into the lower stage cascaded circuit101.

FIG. 10 yet shows another yet another cascaded schematic where bothfunctions of the liquid-suction heat exchanger 132 and economizer heatexchanger 120 are incorporated into the lower stage cascaded circuit101. These enhancement features can be used independently or incombination with each other. This embodiment shows a lower stagecompressor 202 and an upper stage compressor 201.

FIG. 10 also schematically shows a “black box” 300, which illustrates aperformance enhancement feature such as disclosed in any of the aboveembodiments. That is, both circuits can be provided with such a feature.

It should be noted that the performance enhancement features describedabove could be incorporated and operated independently or in combinationwith each other for each of the cascaded circuits within the refrigerantsystem. Also, it has to be understood that there could be more than twocascaded circuits operating within a refrigerant system. Obviously, inmany cases, it would make more sense to apply performance enhancementfeatures listed above to the cascaded circuits charged with therefrigerants that don't operate well in the basic refrigerant cycle.

It should be pointed out that many different compressor types could beused in this invention. For example, scroll, screw, rotary, orreciprocating compressors can be employed.

The refrigerant systems that utilize this invention can be used in manydifferent applications, including, but not limited to, air conditioningsystems, heat pump systems, marine container units, refrigerationtruck-trailer units, and supermarket refrigeration systems.

Although embodiments of this invention have been disclosed, a worker ofordinary skill in the art would recognize that certain modificationswould come within the scope of this invention. For that reason, thefollowing claims should be studied to determine the true scope andcontent of this invention.

1. A refrigerant system comprising: at least one pair of circuitsoperated in a cascade fashion, with a first circuit including a firstcompressor compressing a refrigerant and delivering the refrigerant to aheat rejection heat exchanger, refrigerant passing from said heatrejection heat exchanger through an expansion device, and then to arefrigerant-to-refrigerant heat exchanger, refrigerant from saidrefrigerant-to-refrigerant heat exchanger being returned to said firstcompressor; a second circuit, said second circuit including a secondcompressor, said second compressor delivering refrigerant to saidrefrigerant-to-refrigerant heat exchanger, refrigerant from saidrefrigerant-to-refrigerant heat exchanger passing through an expansiondevice, and then through said heat accepting heat exchanger; saidrefrigerant-to-refrigerant heat exchanger providing heat transfercommunication between said first circuit and said second circuit; saidfirst circuit utilizing a first refrigerant, and said second circuitutilizing a second refrigerant; and at least one additional performanceenhancement feature incorporated in at least one of said first circuitor said second circuit.
 2. The refrigerant system as set forth in claim1, wherein said additional performance enhancement feature is aneconomizer.
 3. The refrigerant system as set forth in claim 2, whereinthe economizer function is provided by an economizer heat exchanger. 4.The refrigerant system as set forth in claim 2, wherein said economizerfunction is provided by a flash tank.
 5. The refrigerant system as setforth in claim 2, wherein a bypass function is also incorporated intothe refrigerant system.
 6. The refrigerant system as set forth in claim5, wherein the economizer and the bypass function are both included inthe first circuit.
 7. The refrigerant system as set forth in claim 5,wherein the economizer and the bypass function are both included in thesecond circuit.
 8. The refrigerant system as set forth in claim 2,wherein the economizer is included in the first circuit.
 9. Therefrigerant system as set forth in claim 2, wherein the economizer isincluded in the second circuit.
 10. The refrigerant system as set forthin claim 1, wherein the additional performance enhancement feature is aliquid-suction heat exchanger.
 11. The refrigerant system as set forthin claim 10, wherein an economizer is also included in the refrigerantsystem.
 12. The refrigerant system as set forth in claim 11, wherein theliquid-suction heat exchanger and the economizer are both included inthe first circuit.
 13. The refrigerant system as set forth in claim 11,wherein the liquid-suction heat exchanger and the economizer are bothincluded in a second circuit.
 14. The refrigerant system as set forth inclaim 10, wherein the liquid-suction heat exchanger is included in thefirst circuit.
 15. The refrigerant system as set forth in claim 10,wherein the liquid-suction heat exchanger is included in the secondcircuit.
 16. The refrigerant system as set forth in claim 1, whereinsaid first refrigerant is hydrocarbon.
 17. The refrigerant system as setforth in claim 1, wherein said second refrigerant is CO2.
 18. Therefrigerant system as set forth in claim 1, wherein said firstrefrigerant and said second refrigerant are the same refrigerants. 19.The refrigerant system as set forth in claim 1, wherein said firstrefrigerant and said second refrigerant are different refrigerants. 20.The refrigerant system as set forth in claim 1, wherein at least onecircuit from said pair of circuits itself is represented by a pair ofcircuits. 21.-42. (canceled)